Fusion cages or interbody fusion devices provide a means of opening the collapsed disc space between opposing vertebrae to relieve pressure on nerves and/or the spinal cord by placing a spacer or spacers in the disc space.
Presently used fusion cages are either cylindrical or rectangular devices having an external threaded or toothed portion for engaging the vertebral end plates in order to prevent the cage from slipping. Such cages are generally hollow. They can be filled with graft in order to induce fusion of the two vertebrae together. Such devices provide great potential for eliminating the large incisions required for posterior instrumentation and open a door for minimally invasive surgery. the large incisions required for posterior instrumentation and open a door for minimally invasive surgery.
The art of spinal implant, such as fusion cages has become highly developed. Recent developments have been directed to various aspects of the cage design. For example, the U.S. Pat. No. 5,055,104 to Ray, issued Oct. 9, 1991, provides a method of surgically implanting threaded fusion cages between adjacent lower back vertebrae by an anterior approach. The device disclosed therein consists of a shape-retaining solid helix of wire that is hollow within the helix and has openings between adjacent turns of the cage.
The U.S. Pat. No. 4,011,602 to Rybicki et al. discloses an expandable device for attachment to bone tissue. The device includes an adjustable mechanism for expanding the body member against tissue in which it is implanted to an extent such as to provide a compressive stress capable of maintaining a snug and stable fit. However, once adjusted, the device is solid and fixed, just as the device disclosed in the Ray patent discussed above.
The U.S. Pat. No. 5,489,308 to Kuslich et al., issued Feb. 6, 1996 also provides a spinal implant for use in spinal stabilization. The implant includes holes therethrough which are positioned to chip bone into the implant as the implant is rotated.
Each of the aforementioned patents, as well as many other patents in the art, address such issues as anterior approach, expansion and then fixation, and means for automatically positioning bone chips within the hollow space therein to promote fusion. However, there remains several issues of maximal importance which are not addressed by the prior art.
For example, fusion devices presently in use are cylindrical or rectangular devices which are large and bulky. Size is an extremely important factor in minimally invasive surgery wherein the device is placed inside the body using a cannula to minimize the incision size and therefore hospital recovery time. In this process, it is desirable to use as small of an incision as possible. A cannula is used to track the implant to its desired destination, the implant having to be smaller in size than the inner core of the cannula to allow insertion therethrough. Present spinal implants require a larger incision due to their large and bulky shape necessitated by their structure thereby not being directed at minimizing the incision.
Another problem with presently used implant cages is a complete lack of understanding of load sharing, Wolf's law, and fusion quality. The above cited patents, as well as others, provide a fixed device which is inserted between the vertebral end plates and filled with graft. It has been previously assumed that if the graft is placed properly, fusion will occur. However, bone quality is related to stress. Wolf's law states that bone grows along lines of stress. The aforementioned prior art cages hold the vertebral bodies apart and act merely as solid spacers. Therefore, the graft material inside the cage resorbs and is never stressed. Without stress, the graft material inside the cage does not effectively provide fusion quality. If a fusion takes place, which is difficult to determine at best in metal cages using X-rays or most other diagnostic techniques, the quality of fusion can be poor at best.
Secondly, as mentioned above, the size of the cage is an important issue. The future of spine surgery will be based upon minimally invasive surgical techniques as discussed above. The surgeon implants the cage or cages into the disc by use of a cannula. This allows for small skin incision to minimize soft tissue trauma and reduce recovery time and hospital stay. To properly relieve the pressure on the nerves and spinal cord, the collapsed disc space should be open to as close as possible to its precollapsed stage. With the present cage designs discussed above, the cage is designed to open the disc space to 16 mm, such cage requiring a cannula of 18 to 20 mm. Such a cannula requires a large incision. It would be desirable to provide a means to allow a 16 mm cage to go down a smaller cannula, such as a 12 mm or 14 mm cannula.
It is further desirable to allow the cage to share the stress with the surrounding bone. Such a dynamic would allow the cage to act not only as a required spacer, but also allow the bone to be stressed to improve fusion success and fusion quality.
It would further be desirable to provide the surgeon with a means of easily opening the disc space. Present day devices require instrumentation to open the disc space, which is often ineffective or difficult to do.